We will study in the kidney: 1. Site and mechanism of action of PTH. 2. Effect of calcitonin. 3. Mechanism of water reabsorption in the proximal tubule. 4. Role of the pelvic urine in the renal concentration mechanism. 5. Elemental composition of isolated segments of nephron. 6. Mechanism of anuria in acute tubular necrosis. Usual methods of clearance and micropuncture and new methods of electron probe analysis of liquid droplet and tissue will be used. BIBLIOGRAPHIC REFERENCES: Lang, F., R. Greger, C. Lechene and F.G. Knox. Micropuncture techniques. In Methods in Pharmacology. Vol. 4B, Ed. M. Martinex-Maldonado, Plenum Publishing Company, New York. (To be published, 1977). Greger, R., F. Lang, F.G. Knox and C. Lechene. Analysis of tubular fluid. In Methods in Pharmacology. Vol. 4B, Ed. Martinex-Maldonado, Plenum Publishing Company, New York. (In press, 1977).